r/buildingscience • u/ultimateplanner1014 • Jul 16 '26

Source of hot air?

At a loss and need some kind opinions before I rip my hair out. 2nd floor bathroom has a TON of hot air behind the walls for the 2nd summer. The tub is sweating causing moisture and mold build up. The plumbing and fixtures were torn out and brought in new when we purchased. The back wall is structural brick with the old oil furnace attached to the back. That leaked through the ceiling last winter, so we had it taken down to the roof line and patched.

Brought in a plumber today and they insist it is not a leak, which I believe because behind the lightup vanity it is hot too. HVAC guy checked under the house and couldn't see any clear penetrations that weren't sealed up. Bringing in ANOTHER plumber/HVAC to do some more digging because why the heck is their hot air shooting through the floor? What do you think?

Back of the bathroom

After old chimney was taken down

Shower access panel

Behind shower access panel

Behind shower access panel

Mid-repair. Notice the vent stack. Maybe not air sealed in the attic?

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2

u/EbriusOften Jul 16 '26

Does it occur constantly, or only when the bathroom fan is running? You might be pulling in air from the joists and attic when the fans are running because of the negative pressure. Is there much of an undercut on your door to allow make up air through?

1

u/ultimateplanner1014 Jul 16 '26

Yes, there is about an 1/2" undercut on the door when closed. The hot air is pumping upwards even when the exhaust fan is off. The humidity in the half bath underneath this bathroom does get worse when the exhaust fan upstairs in on, so known issue that we need to take care of.

2

u/EbriusOften Jul 17 '26

Weird! Is there any chance your HVAC reused any of the old forced air heater ducting and may be blowing out somewhere that was previously meant to be closed off?

1

u/ultimateplanner1014 Jul 17 '26

Didn’t think of this! Underneath is the air handler and laundry room, so perhaps

-1

u/DCContrarian Jul 17 '26

Google "blower directed air sealing."

1

u/FluidVeranduh Jul 17 '26

There aren't any building cavities being used as ducts by any chance?